Why is turbotax asking me to enter the total value of all of my traditional IRA, SEP, and SIMPLE IRA Accounts as of end of tax year?

Do you have any nondeductible contributions to those accounts?   TurboTax is probably asking for information needed for Form 8606 or to calculate how much of a retirement distribution is taxable.  

Any money you contribute to a traditional IRA that you do not deduct on your tax return is a “nondeductible contribution.”    No individual’s money is supposed to be subject to federal income tax twice.  Form 8606 gets it “on the record” that a portion of the money in your IRA has already been taxed.   Later on, when you take distributions, a portion of the money you get back will not be subject to income tax.   

Boosting Retirement

Nondeductible Contributions

The only other reason that TurboTax might ask for your year-end balance in traditional IRAs is if your entries indicate that you have an excess traditional IRA contribution.  Your year-end balance is a factor in determining the penalty for the excess contribution.